1. Field of the Invention
The present invention relates to a piezoelectric ceramic as a material of piezoelectric element applied to a piezoelectric actuator, ultrasonic transducer, and so on, and a method of manufacturing the piezoelectric ceramic, and specifically, to a (lead-free) piezoelectric ceramic containing no lead in the composition and a method of manufacturing the piezoelectric ceramic.
2. Description of a Related Art
Conventionally, a piezoelectric material, which expands and contracts when applied with an electric field and generates an electric signal when applied with pressure, has been widely used. As representative application examples of the piezoelectric material, an actuator in an inkjet head, a transducer for transmitting and receiving ultrasonic waves in an ultrasonic probe, and so on are known.
As piezoelectric materials, piezoelectric ceramics represented by PZT (Pb(lead) zirconate titanate: PbTiO3—PbZrO3), polymer piezoelectric materials represented by PVDF (polyvinylidene-fluoride), and so on are cited. Of the materials, PZT has most widely spread in view of high properties such as piezoelectric constant and electromechanical coupling factor, price, easy handling, and so on.
However, recent years, the toxicity of lead (Pb) contained in PZT has become problematic because lead is hazardous to humans and also causes environmental pollution. Accordingly, the development of a piezoelectric material containing no lead in composition (lead-free piezoelectric material) is being advanced.
Now, a perovskite-type crystal as a main crystal structure of a piezoelectric ceramic such as PZT will be explained. The perovskite-type crystal structure (general formula: ABO3) has a structure in which plural octahedrons formed of oxygen are arranged to share vertices thereof (a corner-sharing structure). Further, elements are located near the center (A-site) of eight octahedrons arranged in that manner and at the center (B-site) of each octahedron. Typically, in an oxide having a high piezoelectric property like PZT, the elements located at the A-sites (A-site elements) include lead. Further, among lead-free piezoelectric oxides, oxides in which the elements located at the B-sites (B-site elements) including niobium (Nb) exhibits a relatively high piezoelectric properties.
As lead-free piezoelectric materials, for example, (Bi0.5Na0.5)TiO3, (K0.5Na0.5)NbO3, KNbO3, and so on are known. However, while typical PZT exhibits the piezoelectric constant d33 of about 400 pm/V to 600 pm/V and relaxor PZT exhibits the piezoelectric constant d33 of about 600 pm/V to 950 pm/V, the lead-free piezoelectric material exhibits at most the piezoelectric constant d33 of about 80 pm/V to 120 pm/V. The piezoelectric constant reaches at the highest to 160 pm/V in a hot-pressed high-density sintered material. Therefore, in the present circumstances, the lead-free piezoelectric material is inferior to PZT in performance and not highly practical.
By the way, study for improving the properties of piezoelectric ceramics has been made principally from the three viewpoints of (1) control of crystal system and composition, (2) optimization of microstructure such as crystal size, voids, internal distortion, and (3) control of crystal orientation.
For example, as an example of the viewpoint (1), Guo et al., “Phase transitional behavior and piezoelectric properties of (Na0.5K0.5)NbO3—LiNbO3ceramics”, APPLIED PHYSICS LETTERS, VOLUME 85, NUMBER 18, 1 Nov. 2004, pp. 4121-4123 discloses that, although the piezoelectric constant d33 of (K0.5Na0.5)NbO3 is about 100 pm/V, the piezoelectric constant of 235 pm/V is obtained by substituting and solid-solving lithium (Li) for the A-side.
Further, as a related technology, Japanese Patent Application Publication JP-P2003-342069 discloses a piezoelectric ceramic composition expressed by the general formula {Lix(K1-yNay)1-x}(Nb1-zSbz)O3, given that x, y, z fall within the composition range of 0≦x≦0.2, 0≦y≦1.0, and 0≦z≦0.2 (except for x=z=0). In JP-P2003-342069, the property of piezoelectric ceramic composition (ceramic) is improved from the viewpoint (1).
On the other hand, Japanese Patent Application Publication JP-A-11-60333 discloses a piezoelectric ceramic including a perovskite-type ceramic containing a rhombohedral crystal as a main phase and a crystal orientation ceramic with {100} face in pseudocubic expression oriented, and further having a degree of orientation of 30% or more according to the Lotgering method. That is, in JP-A-11-60333, the property of the piezoelectric ceramic is improved from the viewpoint (3).
Further, Takenaka, “Grain Orientation of Bismuth Layered-structure Ferroelectric Ceramics and Application to Piezoelectric and Pyroelectric Materials”, Kyoto University Doctor of Engineering Dissertation, 1985, pp. 101-124 shows a relationship between the degree of orientation and the electromechanical coupling factor of the crystal in Bi3.3Pb0.7Ti2.3Nb0.7O12 as a lead-containing piezoelectric ceramic. That is, in the range of the degree of orientation of 60% or more, the electromechanical coupling factor rises steeply with larger degree of orientation.
Therefore, in order to improve the property of a piezoelectric ceramic, control of crystal orientation may have great potential.
As a method of manufacturing an orientation-controlled ceramic, for example, TGG (Templated Grain Growth) method is known. The TGG method is a method of employing plate-like crystal grains having anisotropic crystal structures as a precursor to fabricate a compact, in which grains are oriented, according to the green sheet method, and sintering the compact at normal pressure to obtain crystal sintered material oriented along to the orientation of the plate-like grains.
The green sheet method is a method of obtaining a thick film-like compact by rolling slurry, which has been obtained by mixing organic binder, organic solvent, or the like in ceramic powder, on a carrier film by using a doctor blade device or the like. Employing plate-like grains as ceramic powder in the green sheet method enables fabrication of the compact in which the plate-like grains are oriented by the shear stress at the time of application.
However, in the TGG method, the plate-like grains to be used as a precursor are generally obtained only from crystals with large anisotropy, and therefore, the material system is restrained. Further, any material as a lead-free material with large anisotropy and a large piezoelectric property is not known at present. In the case of using the TGG method in which the crystal structures in plate-like grains are reflected to the final product, fabrication of a piezoelectric ceramic with high performance is difficult.
Accordingly, such TGG method has been improved to develop RTGG (Reactive Templated Grain Growth) method as a technology capable of forming an isotropic perovskite structure. The RTGG method is a method of generating plate-like grains having anisotropic crystal structures such as layered perovskite material, molding the plate-like grains with reactants to fabricate oriented compact containing the plate-like grains, heat-treating it to transform the anisotropic crystal structures into pseudo-equiaxial crystals, and further sintering it for grain growth.
As a related technology, Japanese Patent Application Publication JP-P2002-193663 discloses a sintered material of crystal oriented perovskite-type compound of double oxide expressed by the general formula ABO3, where A is a di-valent metal element and B is a tetra-valent metal element. In JP-P2002-193663, a sintered material composition of a crystal oriented perovskite-type compound as a grain oriented growth structure of a lead-free piezoelectric ceramic is obtained by using an improved RTGG method. Barium titanate is cited as a specific lead-free piezoelectric ceramic.
Further, Saito et al., “Lead-free piezoceramics”, Nature 432, 4 Nov. 2004, pp. 84-87 also discloses a method of manufacturing a piezoelectric ceramic as an improved RTGG method. The method of manufacturing a piezoelectric ceramic includes generating plate-like grains having anisotropic crystal structures (the first precursor) (step 1), heat-treating the plate-like grains with reactants in flux to transform the anisotropic crystal structures into pseudo-equiaxial crystals while keeping the plate-like grains (step 2), molding the plate-like grains of pseudo-equiaxial crystals (the second precursor) with additives to fabricate an oriented compound containing the plate-like grains (step 3), and sintering the oriented compound (step 4).
Furthermore, Saito et al. also discloses that a piezoelectric effect (an electric-field-induced strain) comparable to that of typical actuator-grade PZT is achieved through the combination of the discovery of a morphotropic phase boundary in an alkaline niobate-based perovskite solid solution and the development of a method of highly orienting the polycrystal at direction <001>.
Here, as an anisotropic crystal from which plate-like grains are easily obtained, bismuth (Bi) layered oxide (layered perovskite crystal) is known. The Bi layered oxide refers to a compound containing a sheet-like oxide (perovskite block layer) having an octahedron structure with vertices of oxygen elements and Bi—O layers disposed on and under the oxide. According to the typical RTGG method, the piezoelectric ceramic as the final product inevitably includes the elements contained in the plate-like grains, and thus, when the Bi layered oxide is used as a raw material, only a piezoelectric ceramic containing Bi is obtained.
On the other hand, according to the method disclosed in Saito et al., at step 2, bismuth (Bi) in the crystal structures of the plate-like grains is substituted by sodium (Na) by topochemical conversion. Therefore, even when the Bi layered oxide is used as the first precursor, the final product containing no bismuth is obtained (see FIG. 2 of Saito et al.).
However, in the Bi layered oxide, bismuth is contained not only in the Bi—O layers but also in the perovskite block layer. Accordingly, as disclosed in Saito et al., when the second precursor is produced at step 2, it is unclear whether or not all of the bismuth are substituted by sodium. Further, it is also unclear, when bismuth remains in the second precursor, how the residual bismuth affects the piezoelectric property in the final product.